The present invention relates to a color cathode ray tube, and in particular to a color cathode ray tube provided with an internal voltage-dividing resistor for applying a plurality of different voltages to a plurality of electrodes constituting an electron gun housed in its neck portion, and a conductor for increasing a withstand voltage disposed in a space between the internal voltage-dividing resistor and the inner wall of the neck portion.
A color cathode ray tube used in TV receivers or monitors of information terminals has an electron gun housed within a neck portion of its vacuum envelope for projecting plural electron beams and a phosphor screen (a viewing screen) formed of phosphor elements coated on an inner surface of its panel portion for emitting light of plural colors. A deflection yoke is mounted around the outside of the vacuum envelope for scanning the electron beams from the electron gun on the phosphor screen two-dimensionally to produce a desired image.
In many color cathode ray tubes, a shadow mask serving as a color selection electrode is closely spaced from the phosphor screen such that each of the plural electron beams emitted from the electron gun impinges upon the phosphor elements of its intended color to produce a color image.
For the purpose of improving the quality of a color image over the display screen formed on the phosphor screen, a color cathode ray tube is known which employs an electron gun of the type applying a plurality of high voltages other than its anode voltage to a plurality of electrodes focusing the electron beams.
FIG. 7 is a partially cut-away side view of an essential part of a color cathode ray tube incorporating an electron gun provided with an internal voltage-dividing resistor, and FIG. 8 is a partially cut-away side view of the essential part of the color cathode ray tube of FIG. 7 as viewed in the direction of an arrow A in FIG. 7.
The electron gun for projecting three in-line electron beams is housed within a neck portion 32 of a vacuum envelope 10 of the color cathode ray tube. This electron gun comprises an anode (the sixth grid electrode) supplied with a highest voltage (an anode voltage, 27 kv, for example) 1, an intermediate grid electrode 2 supplied with a voltage (15 kV, for example) obtained by dividing the anode voltage using the internal voltage-dividing resistor, cathodes K (which are supplied with video signal voltages) for emitting the electron beams, a fifth grid electrode group 3 (which are supplied with about 7.7 kV, for example) comprised of plural electrodes constituting a lens for focusing the electron beams emitted from the cathodes K, the fourth grid electrode 4 (which is supplied with 700 V, for example), the third grid electrode 5 (which is supplied with 7.7 kV, for example), the second grid electrode 6 (which is supplied with 700 V, for example), and the first grid electrode 7 (which is grounded, for example). The electrodes 1 to 7 are fixed in the specified order with specified respective spacings therebetween by embedding portions of peripheries of the respective electrodes into a pair of insulating support rods 9.
A shield cup 8 is attached to the sixth grid electrode 1, and ends of electrically conductive springs 11 are welded to a sidewall of a front end of the shield cup 8. A portion of the inner wall of the vacuum envelope 10 is coated with an internal conductive film 10a made of material such as graphite and extending from the funnel portion toward the neck portion. The other ends of the electrically conductive springs 11 press on the internal conductive film 10a such that the anode voltage is supplied to the sixth grid electrode 1 via a high-voltage terminal embedded in the funnel portion.
An internal voltage-dividing resistor 12 of a configuration explained subsequently is attached to an outside surface of one of the insulating support rods 9 facing an inner wall 32a of the neck portion. The internal voltage-dividing resistor 12 is provided with terminals 13, 14 and 15 for electrical connection, the terminal 13 at one end of the resistor 12 is electrically connected to the sixth grid electrode 1 to be supplied with the anode voltage, the terminal 14 at the intermediate position of the resistor 12 is connected to the intermediate grid electrode 2, and the terminal 15 at the other end of the resistor 12 is connected to ground.
The terminal 13 is provided with a connecting tab 13a projecting perpendicularly to the longitudinal axis of the electron gun, and the connecting tab 13a is connected to the sixth grid electrode 1. A connecting tab 14a projects from the terminal 14, and is connected to the intermediate grid electrode 2 to supply thereto a high voltage obtained by dividing the anode voltage by a factor of the ratio of the resistors of the internal voltage-dividing resistor. The terminal 15 is connected to one of stem pins 45 by using an extension of a connecting tab 15a or another member such that the terminal 15 is connected to a potential such as ground potential (hereinafter ground potential) outside the cathode ray tube.
A conductor 16 made of a metal wire is disposed to pass through a space between the inner wall 32a of the neck portion 32 and the internal voltage-dividing resistor 12 and surround the internal voltage-dividing resistor 12 and one of the insulating support rods 9 mounting the resistor 12, and is welded to one electrode of the fifth grid electrode group 3 on opposite sides of the one of the insulating support rods 9.
The conductor 16 is made of nickel or stainless steel of 1 mm in width, for example. A portion of metal contained in the conductor 16 is evaporated by heating the conductor 16 using an external high-frequency induction heater after the completed electron gun assembly has been sealed into the neck portion 32 so as to form a metal thin film 16a on the inner wall 32a of the neck portion, the insulating support rod 9 and the internal voltage-dividing resistor 12 and thereby to produce stable electric potential on the inner wall of the neck portion during operation of the cathode ray tube. Another type of a conductor 16 is also known which uses an extension of a metal wire for connecting together electrodes to be supplied with the same voltage within the cathode ray tube, and still another type of a conductor 16 is also known which has only one of its two ends fixed to the electrode with the other end being not fixed to the electrode.
Reference numeral 17 denotes a conductive film for preventing spark, and the conductive film 17 is a sputtered film of Auxe2x80x94Pd, or Cr, for example, is formed on the surface of the internal voltage-dividing resistor 12 facing the inner wall of the neck portion, and enhances the effects of spot knocking by preventing spark between the conductor 16 and its neighboring electrodes during the spot knocking procedure described subsequently.
FIGS. 9A to 9C are illustrations of the internal voltage-dividing resistor 12 employed in the electron gun of FIG. 7, FIG. 9A is a plan view of the internal voltage-dividing resistor 12 as viewed from its resistance pattern side, and FIGS. 9B and 9C are its side and rear views, respectively.
In the internal voltage-dividing resistor 12, a resistance layer 19 having specified resistance characteristics is formed on one surface of an insulating substrate 18 which is preferably made of ceramic by initially printing a resistance material having desired resistance characteristics such as metal oxide including ruthenium oxide in the form of a desired pattern, and then drying and firing the resistance material.
The pattern (hereinafter also called the resistance pattern 19) of the resistance layer 19 is comprised mainly of plural meandering portions 19a which are located at plural positions and extend meanderingly in a direction of the tube axis (not shown) of the cathode ray tube, a trimming portion 19b, and a linear portion 19c which extends approximately in parallel with the tube axis. Two ends of the resistance pattern 19 are connected to the terminals 13, 15, and its intermediate point is connected to the intermediate terminal 14.
After forming the resistance pattern 19 of this configuration on the insulating substrate 18, a first insulating film 20a made of glass, of a borosilicate lead system, for example, is formed to cover the resistance pattern 19. Similarly a second insulating film 20b is formed over the approximately entire area of the rear surface of the insulating substrate 18 except for regions formed with terminals, and further, a spark-preventing conductive film 17 is coated on the specified portion of the second insulating film 20b. The spark-preventing conductive film 17 is somewhat displaced toward the high-voltage terminal 13 from a portion of the internal voltage-dividing resistor 12 facing the conductor 16. The spark-preventing conductive film 17 is formed by bombarding a target made of Auxe2x80x94Pd or Cr with ions and thereby sputtering Auxe2x80x94Pd or Cr onto the second insulating film 20b covered with a stainless steel mask having an opening of the specified shape.
The terminal 13 formed at one end of the internal voltage-dividing resistor 12 is connected to the sixth grid electrode 1 by the connecting tab 13a projecting from the terminal 13, the terminal 15 formed at the other end of the internal voltage-dividing resistor 12 is connected to an electrode piece at ground potential by the connecting tab 15a projecting from the terminal 15, and the terminal 14 formed at the intermediate position of the internal voltage-dividing resistor 12 is connected to the intermediate electrode 2 by the connecting tab 14a projecting from the terminal 14.
Conductive films (connection leads) 13b, 14b and 15b are provided at and connected to the positions of the resistance layer 19 corresponding to the connecting tabs 13a, 14a and 15a, respectively, and the connecting tabs 13a, 14a and 15a are clamped to the conductive films 13b, 14b and 15b, respectively, as by eyelet-riveting. The conductive films (the connection leads) 13b, 14b and 15b are not covered by the insulating film 20a which covers the resistance layer 19, and therefore they are exposed.
Color cathode ray tubes incorporating internal voltage-dividing resistors of this kind are disclosed in Japanese Utility Model Application Laid-open No. Sho 55-38484 (laid-open on Mar. 12, 1980), and Japanese Patent Application Laid-open Hei 7-94117 (laid-open on Apr. 7, 1995), for example. Further, the development of an electron gun employing an internal voltage-dividing resistor is reported in xe2x80x9cDevelopment of a NEXT Electron Gun for 46 cm 100xc2x0 Narrow Neck Color Display Tubes,xe2x80x9d Technical Report of the Institute of Electronics, Information and Communication Engineers, EID99-99 (2000-01), Electronic Display, Jan. 28, 2000.
It is one of the present invention to provide a color cathode ray tube employing an electron gun provided with an internal voltage-dividing resistor having superior withstand voltage characteristics and capable of providing a high-definition image display.
To achieve the above object, in accordance with an embodiment of the present invention, there is provided a color cathode ray tube including a vacuum envelope having a panel portion with a phosphor screen formed on an inner surface thereof, a neck portion, and a funnel portion connecting the panel portion and the neck portion, and an electron gun housed in the neck portion, the electron gun comprising: an electron beam generating section; a plurality of focus electrodes; an anode, the electron beam generating section, the plurality of focus electrodes, and the anode being arranged in the order named, and fixed in predetermined axially spaced relationship by a pair of insulating support rods for focusing three electron beams emitted from the electron beam generating section onto the phosphor screen; a voltage-dividing resistor for producing an intermediate voltage to be applied to a first one of the plurality of focus electrodes adjacent to the anode by dividing a voltage applied to the anode, the voltage-dividing resistor being disposed in the vicinity of a surface of one of the pair of insulating support rods on a side thereof facing toward an inner wall of the neck portion, the voltage-dividing resistor comprising an insulating film, a resistance pattern, and an insulating substrate disposed in the order named from the insulating film toward the inner wall of the neck portion; and a metal conductor surrounding the voltage-dividing resistor and the one of the pair of insulating support rods and fixed to a second one of the plurality of focus electrodes, the second one of the plurality of focus electrodes being disposed upstream of the first one of the plurality of focus electrodes in a path of the three electron beams, wherein the resistance pattern is such that a potential difference between the metal conductor and a portion of the resistance pattern facing the metal conductor is equal to or smaller than 4 kV.
The present invention is not limited to the above configuration, and various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.